1. Field of the Invention
The present invention relates to a station selecting apparatus. More specifically, the present invention relates to a preset circuit for presetting a channel signal in a station selecting apparatus of a voltage synthesizer type in a television receiver, for example.
2. Description of the Prior Art
FIG. 1 is a block diagram showing one example of a tuner of a television receiver which constitutes the background of the invention. Since such television tuner is well known to those skilled in the art, the same will be described in brief to the extent required for description of the present invention. A tuner 100 is connected to a VHF antenna 1 and UHF antenna 2. A television signal received by the VHF antenna 1 is applied to a VHF high frequency amplifier 101 to be amplified and the output therefrom is applied to a VHF mixer 103. The VHF mixer 103 serves to mix the VHF television signal with a local oscillation signal obtained from a VHF local oscillator 105, thereby to convert the same into a VHF intermediate frequency signal. On the other hand, a UHF television signal from the UHF antenna 2 is applied to a UHF high frequency amplifier 102 to be amplified and the amplified output is applied to a UHF mixer 104. The UHF mixer 104 serves to mix the UHF television signal with a local oscillation signal obtained from a UHF local oscillator 106, thereby to convert the same into a UHF intermediate frequency signal. The UHF intermediate frequency signal is amplified by a UHF intermediate frequency amplifier 108 and the output therefrom is applied to the VHF mixer 103. Accordingly, the VHF mixer 103 serves as a UHF intermediate frequency amplifier on the occasion of reception of the UHF band. The output obtained from the VHF mixer 103 is applied to an intermediate frequency amplifier circuit of a subsequent stage, not shown. Each of the VHF local oscillator 105 and the UHF local oscillator 106 comprises a voltage controlled variable reactance device such as a variable capacitance diode for the purpose of selecting a desired channel of a desired frequency band. To that end each of the VHF local oscillator 105 and the UHF local oscillator 106 is supplied with a tuning voltage Vt from a channel selecting apparatus 200. At the same time each of these circuits 105 and 106 is supplied with band selecting voltages BL, BH and BU from the apparatus 200.
FIG. 2 is a block diagram showing one example of the channel selecting apparatus 200 which constitutes the background of the invention. The apparatus 200 is structured to be of a voltage synthesizer type and is also structured to be presettable. The present invention is directed to an improvement in a channel selecting apparatus of a presettable voltage synthesizer type. Although such channel selecting apparatus 200 is well known to those skilled in the art, the same will be briefly described to the extent required for description of the present invention. A control unit 21 is implemented by an integrated circuit of such as Model No. TC9002, manufactured by Tokyo Shibaura Electric Co., Ltd., Japan. A memory 22 is connected to the control unit 21 for the purpose of storing a channel signal being preset by a preset circuit 23. The memory 22 may be implemented by a non-volatile memory MNOS of such as No. TMM 841 manufactured by Tokyo Shibaura Electric Co., Ltd., Japan, whereby a stored content is kept stored even after a power supply is turned off. The memory 22 serves to store not only a channel signal but also band information of the UHF band, VHF low band or VHF high band. The preset circuit 23 comprises a charging/discharging circuit including a parallel connection of a variable resistor 231 and a capacitor 232. A charging/discharging operation of the charging/discharging circuit is controlled by a switching field effect transistor 233 included in the control unit 21. More specifically, a switching pulse as shown in FIG. 3A is applied to the field effect transistor 233, so that the field effect transistor 233 is turned on during the period T1 of the switching pulse and the field effect transistor 233 is turned off during the period T2 of the switching pulse. Accordingly, during a time period when the field effect transistor 233 is turned on, the capacitor 232 is charged with a current from a voltage source Vdd and the electric charge in the capacitor 232 is discharged to a reference potential, i.e. the ground through the variable resistor 231 during the time period when the field effect transistor 233 is turned off. Accordingly, the terminal voltage of the capacitor 232 changes as shown in FIG. 3B. The terminal voltage of the capacitor 232 is sliced with a proper threshold value Vs by means of a slicing circuit, not shown. Accordingly, a channel signal 234 obtained from the preset circuit 23 is obtained as pulse shaped as shown in FIG. 3C.
Contacts L, H and U of a band switch 24 are further connected to the control unit 21, so that these contacts L, H and U may correspond to the VHF low band, VHF high band and UHF band, respectively. A common contact of the band switch 24 is connected to an appropriate voltage, say 10 V, through a diode for preventing a reverse directional current. Accordingly, by selecting a particular contact by the band switch 24, a corresponding band is selected. A memory switch 25 is connected to the control unit 21. The memory switch 25 is operated to write the channel signal 234 obtained from the preset circuit 23 and the band signal obtained from the band switch 24 in the memory 22 on the occasion of the preset mode.
A channel selecting switch circuit 26 and a channel selection indicator circuit 27 are further connected to the control unit 21. The channel selecting switch circuit 26 comprises a plurality (n) of channel switches 261, 262, . . . , 26n and the indicator circuit 27 comprises a plurality (n) of indicator lamps 271, 272, . . . , 27n. The indicator lamps 271, 272, . . . , 27n each may be a light-emitting diode. Each of the light-emitting diodes 271 to 27n is responsive to operation of a corresponding one of the switches 261 to 26n to be supplied with a driving voltage of an appropriate voltage say 10 V, thereby to indicate that the corresponding channel is selected.
Now an outline of the preset mode of the FIG. 2 channel selecting apparatus 200 will be described. Any one of the channel switches 261 to 26n corresponding to a channel which is to be preset is first turned on. As a result an address of the memory 22 where preset data is to be written is designated. Then a band out of the VHF low band, VHF high band and UHF band to which the desired channel pertains is set by the band switch 24. The band information is entered to the control unit 21 from an appropriate input terminal. The variable resistor 231 is adjusted to attain a picture of the best quality in the desired channel. More specifically, the discharging time T (FIG. 3) is changed by adjusting the variable resistor 231, whereby a tuning operation is effected to the desired channel. Then the memory switch 25 is operated. Accordingly, the band information from the band switch 24 as well as the channel signal 234 is stored in the address of the memory 22 designated by any one of the channel switches 261 to 26n. Meanwhile, the channel signal 234 may be written in the memory 22 as a binary code corresponding to the pulse width T (FIG. 3). Thus the preset operation is completed.
Then a tuning voltage Vt is applied to the tuner 100 (FIG. 1) from the channel signal and the band information preset in the memory 22, while a tuning voltage generator 29 and a band selection voltage generator 28 are connected to the control unit 21 for the purpose of providing a band selecting voltage BL, BH or BU. The tuning voltage generator 29 is supplied with a PWM signal read from the memory 22 (which is a signal including channel information in pulse width as in the case of the channel signal 234 shown in FIG. 3C). The band selecting voltage generator 28 is supplied with band information read from the memory 22. Now referring to FIGS. 4 and 5, the tuning voltage generator 29 will be briefly described. The tuning voltage generator 29 comprises a voltage converting circuit 291 and a low pass filter 292. The PWM signal from the control unit 21 is applied to the voltage converting circuit 291 as shown in FIG. 5A. The voltage converting circuit 291 converts the PWM signal to a predetermined value Vz by means of a zener diode, thereby to provide a voltage signal as shown in FIG. 5B. The voltage signal is smoothed by means of a low-pass filter 292, thereby to provide a direct current tuning voltage Vt as shown in FIG. 5D. Meanwhile, FIG. 5C shows a voltage wave form at the point 252a of the low-pass filter 292. Such tuning voltage generator 29 is also well known to those skilled in the art.
The tuning voltage Vt obtained from the tuning voltage generator 29 is applied to the tuner 100 shown in FIG. 1, whereby the tuner 100 is tuned to a desired channel corresponding to the tuning voltage Vt.
On the other hand, it has been known that in Europe, for example, the tuner 100 is generally adapted to exhibit a characteristic as shown in FIG. 6. More specifically, in Europe the standard system has been adopted in which the VHF low band comprises Channels E2 to E4, the VHF high band comprises Channels E5 to E12 and the UHF band comprises Channels E21 to E69. In a tuner adaptable for such standard, the receivable lowest frequency in the VHF low band, i.e. Channel E2 is set to be capable of receiving the channel in the case where the tuning voltage is 3 V, for example. However, a television tuner must be able to select assuredly Channel E2 under any conditions. Therefore, conventionally the channel selecting apparatus 200 (FIG. 2) has been adapted to provide to the tuner 100 (FIG. 1) a sufficiently small tuning voltage Vt as small as say 0.2 to 0.3 V as compared with the above described 3 V. Accordingly, a conventional television tuner has in an ordinary state a receivable frequency range extended as shown by the dotted line in FIG. 6 toward a lower frequency region over the required receivable frequency range. Likewise, the other bands have also been designed such that the required channels can be assuredly received with a considerable allowance.
However, recently it has come to be legally required that a receivable frequency range is very narrowly restricted for the purpose of effective utilization of an electric wave and maintenance of secrecy in communication. In West Germany, for example, FTZ provides that an allowable range of shift outside the frequency range over the upper limit and the lower limit of each frequency band must be basically smaller than 300 kHz, with an exception that a shift outside the frequency range by 8 MHz in the upper limit and by 7 MHz in the lower limit is merely allowed in the receiving frequency band of 47 MHz to 800 MHz. Likewise, as regards the television broadcasting in Canada as shown in FIG. 7, DOC requires that any frequency regions lower than channel A7 and higher than channel A13, except for CATV Channels I and J, must not be receivable by ordinary television receivers.
Under the circumstances as a simple approach to satisfy the above described strict regulations, one might think of adjusting the respective tuning elements, not shown, so as to make the lower limit frequencies of the respective bands aligned with the lower limit of the variable range of the tuning voltage and to make the upper limit frequencies of the respective bands aligned with the maximum value of the variable range of the tuning voltage. However, generally in mass production of television tuners a diversification is caused in a receiving frequency characteristic with respect to the tuning voltage for each of the tuner sets. More specifically, referring to FIG. 6, as regards the points a to f at the upper end and the lower end of the respective frequency bands U, H and L, diversifications are caused in the range as shown as A to F, respectively. On the other hand, the variable range of the tuning voltage Vt also involves a diversification for each of the channel selecting apparatuses. Therefore, the above described adjustment of the tuning frequency cannot be made only in a tuner itself and accordingly such adjustment must be made of the tuner in combination with a channel selecting apparatus. Such adjustment is extremely tiresome in manufacture. In addition, a problem is involved that since any channel signal can be set through adjustment of the variable register 231 (FIG. 2) in a presettable tuner to which the present invention is directed even a frequency which must be unreceivable is made receivable in a television receiver when an operator sets a channel signal outside the legal restriction as described above. In other words, in case of such presettable station selecting apparatus, even such possibility of presetting by an operator need be taken into consideration. Thus, any of the conventional approaches could not achieve the effect to the satisfaction.